1. Field of the Invention
The present invention generally relates to large scale integrated (LSI) semiconductor circuit manufacturing processes and, more particularly, to the formation of high quality, high yield metal silicide films used to form diodes and/or to make ohmic contacts.
2. Description of the Prior Art
Platinum silicide (PtSi) is traditionally used for low ohmic contact resistance and for high barrier Schottky barrier diodes (SBDs), infrared (IR) light sensitive diodes, and the like. The basic steps in forming PtSi involve:
a) Non-photo galvanic etch for silicon contact cleaning; PA1 b) Pt deposition; PA1 c) 550.degree. C. N.sub.2 sinter; and PA1 d) Aqua regia etch to remove unreacted Pt and a sulfuric acid based cleaning to remove any carbonaceous residue. PA1 a) By evaporation of Pt with a 350.degree. C. substrate temperature. This technique has shown advantages in producing complete PtSi, less sinter condition dependence and clean PtSi/Si interface with excellent SBD characteristics. Disadvantages are: (i) Limited lateral spreading giving an annular ring devoid of Pt; (ii) Long cycle time due to slow cool down; (iii) PtSi creep along sidewall; and (iv) Pt nodule formation. PA1 b) By sputter deposition at room temperature. The advantage is less cycle time. Disadvantages are: (i) Extreme sensitivity to sinter condition; (ii) Incomplete PtSi conversion giving missing PtSi and low forward voltage (V.sub.f) SBD; (iii) PtSi creep along walls; and (iv) Pt nodule formation.
For Pt deposition, two techniques are used:
For contamination reduction, better process control and automation, a sinter furnace is used where the sinter tube is isolated from atmosphere with circulating N.sub.2. This reduces O.sub.2 in the sinter tube whereby no protective SiO.sub.2 layer is formed on the PtSi. This results in a massive missing PtSi problem. Addition of any O.sub.2 in the N.sub.2 results in incompletion of the PtSi reaction and again results in similar massive missing PtSi. A similar problem has been observed with an open sinter tube when there is any deviation in the sinter process or a crack in the tube or even poor vacuum in sputter tool.
The sinter process actually consists of two silicide formation reactions steps and a passivation oxide formation step, namely: EQU 2 Pt+Si.fwdarw.Pt.sub.2 Si EQU Pt.sub.2 Si+Si.fwdarw.2 PtSi EQU Si (from PtSi)+O.sub.2 .fwdarw.SiO.sub.2 (passivation oxide)
Since these reactions take place simultaneously and the first two compete with the third, incomplete PtSi formation results. Poor passivation also results in scalloping, or missing PtSi, after the aqua regia etch.